1. Field of the Invention
The present invention is in the field of optical information recording media which use a laser beam for recording or writing information thereon, and is more particularly concerned with a reflective metal layer used in such optical information recording media.
2. Description of the Prior Art
There are several types of optical information recording media, including the ROM type such as a video disk, a compact disk, or the like, which is capable of only playing back information recorded thereon, the so-called WO type (write-once-disk) type which can record information thereon and from which it can be played back, or a rewritable optical recording disk, a magneto-optical record disk, or the like, capable of reading information or recording the same by using a laser beam. In such optical information record media, a reflective metal layer is formed on the back surface of the recording layer or surface.
For example, in an information record medium of the ROM type shown generally in FIG. 1, there may be provided a video disk of a double-sided type having one major surface consisting of a transparent substrate 1 composed of polycarbonate resin or the like, on which there are formed information pits 2, for example, consisting of concave portions corresponding to recorded information. On the surface of the substrate 1 on which the information pits 2 are formed, there is deposited a reflective metal layer 3 composed of an aluminum film which has been deposited by physical vapor deposition processes such as vacuum evaporation or sputtering. On the reflective metal layer 3 there is coated a protective film 4 composed of an acrylic resin cured, for example, by ultraviolet irradiation. A pair of such substrates 1 each having the information pits 2 as described above are bonded together at their protective films 1 by means of an adhesive agent 5. A laser light is irradiated upon the respective information pits from the back surfaces of the substrates 1 with the reflective pits as shown by an arrow a or b and the presence or absence of the pits 2 is detected, i.e., the information is read by detecting the reflected light from the respective reflection metal layers 3.
However, in an information recording medium of this type having a reflective metal layer made of aluminum, its optical characteristics are sometimes deteriorated, and loss increases upon reading the information. In the WO type or rewritable information record medium, the writing and reading characteristics are occasionally considerably deteriorated. The reason for this is that the reflective metal layer 3 composed of aluminum may become corroded, lowering its reflectivity and it may be recrystallized so as to deteriorate its planar property. For example, although the corrosion-proof properties of aluminum can be improved to a certain degree by using an aluminum having a high purity, its recrystallization cannot be avoided.